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Патент USA US3031347

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United States
Patented Apr. 24, 1962
solution of chromic acid and a reducing agent which is
3 031 333
compatible with the chromic acid. This compatibility
means that the chromic acid and the reducing agent, even
though both are present in the solution, will not in the
appropriate dilution react rapidly with each other and will
not form a visible precipitate while the solution is being
to Pennsalt Chemicals Corporation, Philadelphia, Pa.,
contacted with the steel.
a corporation of Pennsylvania
The steel sheet wet with the solution is then dried at
N0 Drawing. Filed June 20, 1956, Ser. No. 592,552
a temperature above 212 degrees F. During this drying,
2 Claims. (Cl. 117-50)
10 the chromic acid reacts with the reducing agent and
This invention relates to coated steel members, more
becomes partially reduced. As va result, the ?nal coating
particularly to such members which are protected from
is a combination of hydrated chromium oxides containing
about 20 to 60% chromium by weight.
Among the objects of the present invention is the provi
An inordinately high degree of corrosion resistance is
sion of novel coated steel members which are simple and 15 obtained when 40 to 95% by weight of the chromium in
inexpensive to make and yet are extremely resistant to
the ?nal coating is in What corresponds to- the trivalent
condition, the remainder being hexavalent, and the coat
Additional objects of the present invention include the
ing itself weighs about 10 to 200 milligrams for every
provision of a novel method for making such members.
square foot of surface that it covers. Coatings of less
Ludwig K. Schuster, Philadelphia, and Alfonso L. Baldr,
Jr., Drexel Hill, Pa., assignors, by mesne assignments, .
The above as well as still further objects of the present
invention will be more clearly understood from the
following description of several of its exempli?cations.
Plain carbon steels such as ordinary SAE 1010 steel,
weight give less protection, although coating weights need
not be more than about 30 milligrams per square foot to
the canning industry, are relatively inexpensive and
give all the protection that is needed.
If the reducing agent is omitted from the coating bath,
dried coatings of the above weight range will not show
the desired trivalent chromium content. They also tend
simple to fabricate and use in practically any desired con
to be somewhat deliquescent and therefore inferior.
or the conventional Blackplate sheets that are used in
Unfortunately, these steels =are extremely
Typical reducing agents suitable for use in connection
with the present invention are organic polyalcohols such
as sugars, including invert sugar, sucrose, dextrose, glycol
vulnerable to corrosion. In fact, unless the steel is pro
tected as by a ?lm of oil, it will usually corrode even
before it can be delivered by the manufacturer to the 30 and polyethylene glycols, glycerine, mannitol, sorbitol,
triethanolamine, hydroxylamine salts such as its sulfate
However, by reason of the low cost and extreme versa
and hydrochloride, phosphorous acid, and potassium
tility of these steels, many different types of treatment
have been developed to reduce their corrosion so that
Care should be taken to limit the amount of water
their use can be extended to such ?elds as canning and 35 soluble material that is included in the coating. Al
the like. Perhaps the most widely spread corrosion-re
though as much as 5 or 6% of such materials can gen
sistant treatment is the application of a tin plating. Al
though the plating of tin has proven to be very satis
erally be tolerated, it is preferred to use reducing agents
that do not leave such materials in the ?nal product.
Potassium iodide is not a preferred type of reducing agent
factory in most respects, it is fairly expensive and requires
in this country that the tin be imported from abroad.
40 inasmuch as it is carried through as Water-soluble potas
According to the present invention, very effective corro
sium compounds after reducing the chromic acid. Phos
sion resistance can be imparted to members made of the
phorous acid is a better reducing agent even though it is
above steels, particularly where these members are to _ oxidized to phosphoric acid because the drying operation
come into body-engaging contact, by applying a coating
converts such phosphoric acid to water-insoluble phos
of an in situ formed combination of hydrated chromium 45 phates. The most effective reducing agents appear to be
oxides containing about 20 to 60% chromium by weight,
the organic polyalcohols since they are readily oxidized
about 40 to 95% of the chromium by weight being tri
at the higher temperatures and are fairly stable at the
valent, the remainder being hexavalent, and the coating
lower temperatures.
weighing about 10 to 200 milligrams per square foot of
As indicated above, the coatings of the present inven
surface that it covers. The steel is preferably subjected 50 tion are preferably applied to a steel that has had its
to a grain-boundary etch, or is oxidized so that its surface
surface subjected to a grain-boundary etch or covered
is covered with a ?lm of uniformly adherent iron oxide,
with an oxide ?lm. The etching may vary from an in
before the coating is applied.
signi?cant amount to a heavy etch that removes 100 to
A can of lubricating oil or roasted coffee beans can
1000 milligrams of metal per square foot of steel surface.
have its inner surface covered with the ‘above coating ‘to 55 With etching of more than about 400 milligrams per
prevent rusting prior to ?lling as well as when ?lled with
square foot, the best results are not obtained unless the
oil or coffee beans; the outer surfaces of the can can also
mixed chromium oxide coating weighs more than about
have the same type of combined chromium oxide coating
30 milligrams per square foot. The oxidation can be
which provides an excellent substrate for subsequent litho
effected by merely attacking the surface with a reagent
graphic coatings, varnishes, lacquers, enamels and other 60 that converts it to oxide with substantially no removal
organic coatings.
Instead of applying the coating of the present inven
of metal.
For etching the steel surface can be contacted with
tion to the can after it is manufactured or after it is ?lled
aqueous nitric acid having a concentration of from 1 to
and sealed, it is simpler to make the can from steel sheets
20% HNO3 by weight for a period of from 2 to 70 sec
that have the coating applied while they are being pro 65 onds at a temperature of from 60 to 150° F. Aqueous
duced. This permits the coated sheets to be stored, if '
solutions of ammonium persulfate, picric acid or ferric
desired, and even shipped over substantial distances with
nitrate are also effective and react similarly to nitric acid
out requiring any supplemental treatment to prevent them
solutions. For example, an aqueous solution containing
from becoming unmarketable or unsightly as a result of
4% of ammonium persulfate will, when applied at 75° F .
70 for 15 seconds, remove approximately 380 milligrams of
The sheets are readily coated by ?rst making sure they
metal per square foot and provide a very satisfactory sur
are very clean and then passing them through an aqueous
face for the chromium oxide coating. A 11/z% picric
acid solution in water at 155° F. for 12 seconds removes
The same coated sheet can be used for making both
the top and bottom covers of a can, although in some
cases different thicknesses of metal can be used in the
different portions, so that separate coated sheets are re
about 370 milligrams of metal per square foot and is also
very satisfactory. Aqueous ferric nitrate in a concentra
tion of about 5% based on anhydrous material, used at
80° F. for 15 seconds removes about 400 milligrams per
After the can body is secured together, the cover on
square foot and also leaves an excellent pretreatment
one end can be applied in any convenient manner such
surface. The oxidation can be accomplished with hot
as the one generally used in the industry. The can can
concentrated aqueous solutions of sodium hydroxide con
then be ?lled and the remaining cover applied with the
taining sodium nitrate as used in standard bluing opera
tions. A concentration of 39% NaOH and 2% NaNOB 10 usual precautions in the case where the contents have to
be sterilized or heated, or kept in a special atmosphere
used at 280° F. is very effective although any bluing treat
such as under evacuation or superatmospheric pressures.
ment appears to be suitable.
The cans having the coatings of the present invention are
It is not desirable to have oxide pretreatment ?lms that
particularly useful in storing such materials as dried
are loose or non-adherent; the oxide produced should be
foods, nuts, spices, dough mixtures, etc., as well as the
continuous and adherent to the metal surface. However,
above-mentioned coffee and lubricating oil. Wet-packed
such ?lms as ordinary corrosion do not interfere with the
foods or other materials can also be stored in the cans
application of the coating since loose portions are readily
of the present invention, but here it is desirable to have
removed as by a conventional pre-cleaning operation.
The proportion of reducing agent to chromic acid
should be insufiicient for the complete reduction of all
the chromic acid to trivalent condition. The minimum
amount of reducing agent is somewhat below that which
will stoichiometrically reduce the lowest proportion of
the chromium, inasmuch as some of the chromium is re
an organic covering layer (enamel, lacquer, varnish) ap
plied over the coating of the present invention on the
inside surface of the can.
The coatings of the present invention contribute a sub
stantial amount of increased adhesion and wear resistance
for such organic coatings as well as resistance to cor:
duced during the drying operation even if the reducing 25 rosion. Suitable organic layers are those usually loosely
referred to in the art as enamels, sanitary enamels or lac
agent is not present in the bath. Chromic acid solutions
quers such as the oleo-resinous phenolic or vinyl resin
of any concentration can be used and the coating weight
varnishes. Particularly effective forms of such organic
adjusted by controlling the amount of solution that is
type coating are described in U.S. Patents Nos. 2,231,
left on the metal surface when it is being dried.
The ?nal heat treatment is somewhat more effective if 30 407, 2,299,433, 2,479,409 and 2,675,334. Such top coat
ings will even further reduce corrosion as well as increase
carried out at temperatures substantially above 212 de
the wear resistance and lower the contamination of the
grees F. Temperatures between 250 degrees F. and 350
can contents by the oxides.
degrees F. are preferable and provide the most corrosion
The advantages of the present invention are contributed
resistant and adherent forms of coating. At about 450
degrees F. and higher, however, the coating appears to 35 to any plain carbon steel, that is steel that contains no
more than about 2% of alloying metals. They can have
be adversely affected.
a carbon content varying from extremely low values,
In accordance with the above, cans can be made from
0.05% or even less, to as much as 1.4% or higher. The
unfabricated full bright ?nished sheet steel 10 mils thick
phosphorus and sulphur contents can range from sub
by subjecting the sheet to the following operations:
40 stantially zero up to several tenths of a percent. Gen
A. Clean the sheet cathodically in an aqueous solution
erally phosphorus maxima are about 0.15% and sulphur
containing 16 grams KOH per liter using a current
maxima about 0.3%. These materials include the steels
density of 15 amperes per square foot of cathode at
ordinarily considered as carbon steels (SAE 1010 to
140—160° F. for 10 seconds.
1095), free cutting steels, plain carbon tool steels, in-
. Cold water rinse.
cluding those that have up to several percent of silicon,
and casting metals.
grams KOH per liter using a current density of 15 am-t
The inorganic mixed chromium oxide coating of the
peres per square foot of anode at l40~160° F. for 10
present invention is even further improved if ‘after the
?nal high temperature drying it is subjected to a quench
. Cold water rinse.
50 that rapidly reduces its temperature at least about 25°
. Flood with a passivity-preventing 1/2 % aqueous H2804
F. Any liquid appears to be suitable for this purpose,
solution by weight ‘1-2 seconds at 80° F.
and plain or tap water is very effective. The addition
. Cold water rinse.
of 0.03 to 1.0% CrO3 in the quenching Water even
. Spray with an aqueous solution containing 2% nitric
further improves its effectiveness.
acid, 80° F., 8 seconds using a spray pressure of
The mixed chromium oxide is also improved by in
about 6 pounds per square inch.
cluding with these oxides a small amount of an oxide of
. Clean anodically in an aqueous solution containing 16
. Cold water rinse.
Desmut by brushing in water to remove loose or
non-adherent reaction products including any devel
oped in G.
1. Flood with an aqueous solution containing 4% chro
mic acid and 1.3% cane sugar at 75° F. for 2 seconds.
K. Roll through rubber rolls wetted with the ?ooded so
L. Cure by passing the resulting ?lmed metal through a
a metal such as molybdenum that is in the same periodic
group as chromium. Such supplemental oxide can be
added to the chromic acid bath as water~soluble oxides,
60 acids, or as water-soluble salts. It is preferred to add
molybdenum as M003. The advantages of the supple
mental oxides are felt when they make up as much as
1/3 the weigh-t of the ?nal coating, although it is preferred
to use from 5 to 20%.
The combined oxide coatings
show particularly good adhesion to thestecl with or with
drying unit having a set of ceramic gas burners heated
red hot by burning gas, a ?ve second exposure to the
out a previous oxidation, ‘as Well as better adhesion to
incandescent units being used, and the metal reaching
other metals such as titanium and vanadium can also be
an organic top layer. Similarly reducible compounds of
a temperature of 300 to 350° F.
used. In general, it is desirable to diminish the propor
The addition of a wetting agent such as 0.005% of the 70 tion of reduced chromium oxide in the final coating, when
a supplemental oxide is used, to not over approximately
product made by condensing 3 mols of ethylene oxide
with p-(n-octyl) phenol, as described in U.S. Patent No.
The inclusion of 1 to 5% of insoluble or slightly solu
2,115,192, improves the Wetting in step I.
ble ohromate of such divalent metals as zinc, calcium and
If the metal is in the form of an elongated strip, it may
75 strontium also improves the quality of the coating in a
be coiled up directly after step L.
similar manner.
This effect is obtained with or without
the supplemental oxide.
The improvements contributed by the quenching as
described ‘above are also produced with the coatings that
have the supplemental oxides, or the slightly soluble
chromates, or both.
The above coating techniques can be readily carried
out either in a batch process or continuously. They
may, for example, be added at the end of a standard
well as reduces friction to simplify fabrication operations
such as stamping, bending, etc.
Under some conditions the nitric acid treatment of the
steel does not produce the desired effect unless the steel
is subjected to a preliminary activation. Under these
conditions the steel appears to be passive, and any chemi
cal attack as by very dilute sulphuric acid or even me
chanical working of the steel will su?ice to activate it.
The addition of 1/2 to 2% urea to the nitric acid bath
sheet steel production line. In fact, the coating treat 10 tends to suppress NO formation, thereby reducing the
ment of the present invention can be carried out with
the steel moving at a relatively high speed through the
necessary treating stations. Under some conditions, par
ticularly Where the steel is moving very rapidly through
a chromic acid bath, it is desirable to have a wetting 15
tendency for iron to be oxidized to ferric form. The
urea is consumed in the process and can therefore be
replenished either on a continuous basis or by infrequent
As stated previously, etching and oxidizing with HNO3
agent present in the bath. This enables the bath liquid
can be effected with concentrations of HNO3 from 1 to
to more rapidly and uniformly wet the surface of the
20% and temperatures from 60 to 150° F. Not only is
metal. Wetting agents of any type can be used so long
it desirable from a handling and economic standpoint to
as their wetting action is not completely destroyed by the
work with low concentrations of HNO3 (less than 5%
oxidizing action of the bath. Anionic, cationic or non 20 by weight) and low temperatures (less than 110” F.) but
ionic types of wetting agent used in amounts of about
extremely advantageous from an operational standpoint.
0.001 to 0.2% by weight of the bath are effective to
The lower concentration and temperature provide some
keep from developing coating irregularities apparently
what more uniform results and greatly minimize the
due to air bubbles trapped on the metal while moving
tendency for oxidation of ferrous iron to ferric iron. It
through the chromic acid bath. A highly elfective ex 25 should be noted that this oxidation consumes excessive
ample of wetting agent is the polyoxyethylene ether of
amounts of nitric acid and is accompanied by the genera
alkylated phenols such as those produced by condensing
tion of large quantities of noxious nitrogen oxide fumes.
dodecyl phenol with twelve molecules of ethylene oxide.
Other articles with which the invention is very effec
Reference is also made to U.S. Patents 1,970,578 and
tive are shown in U.S. Patent 2,773,623, granted Decem
2,085,706 for more speci?cally disclosed wetting agents 30 ber 11, 1956.
that are suitable.
The coatings of the present invention can be applied to
Another example of the coating process of the present
articles after they are formed as by shaping, stamping, or
invention is as follows:
even casting. When treating such formed articles with
A. Sheets of 14 mil thick SAE 1010 steel are cleaned by
the liquid coating solutions, however, care should be
immersing for ?ve seconds in a 180° F. aqueous solu 35 taken to see that the layer of solution on the article does
not concentrate as by running or dripping to any portion
tion of 5% disodium phosphate and 3% sodium car
of the article that should be protected. A localized thick
. The cleaned sheets are rinsed with water at 70° F.
layer of solution tends to form on the lower edges of the
. Immerse the rinsed sheets in 1/10% solution of sul
article under the in?uence of gravity, but a blast of air
40 can be used to redistribute the thick layer. Alternatively,
furic acid in water ‘for one second at 70° F.
. Rinse again in water at 70° F.
the articles can be rotated or kept moving so as to keep
. Subject the resulting sheets to a uniform action of jets
of an aqueous solution containing 1.2%nitric acid at
a thickened film from forming at any location.
80° F. for two seconds, the jets impinging at a velocity
Although it is indicated above that coating weights of
at least ten milligrams per square foot are required to give
of ten feet per second.
45 the exceptional corrosion resistance for some purposes,
. Rinse the acid treated sheets in cold water at 70° F.
such as to anchor organic covering layers, coatings of
. Pass the sheets between rotating brushes having stain
somewhat lower weight will provide unusually good re
less steel bristles, to remove loose material.
H. The rinsed sheets are sprayed with an aqueous solu
The Weight of the coating is readily determined by dis
tion of 31/2% chromic acid, 1% pentaethylene glycol 50 solving it in the flux or in a strong alkaline solution such
‘and 0.1% of the wetting agent made by condensing
3 mols of ethylene oxide with p-(n-octyl) phenol, as
described in U.S. Patent No. 2,115,192. This solu
tion was prepared in the mixing nozzle by supplying
as 20% NaOH in water at 180° F. and weighing the metal
before and after. The solution thus made can have its
hex-avalent chromium content determined by titration with
sodium thiosulfate. The total chromium content can be
it with two separate streams, one being an aqueous 55 determined by oxidizing another portion of the solution
solution of the chromic acid, and the other an aque
with sodium peroxide and then again titrating with so
ous solution of the remaining ingredients.
I. The sheets carrying the above solution are passed
dium thiosulfate.
Alternatively, the coating can be
scraped off, dissolved and analyzed.
through an air oven, the inside of which is held at
The present application is a continuation-in-part of ap
800° F., the surface of the sheets reaching a tem 60 plication Serial No. 433,698, ?led June 1, 1954, now
perature of 375° F.
Patent No. 2,773,623, granted December 11, 1956. That
J. The hot sheets are quenched
water at 70° F.,
application is in turn a continuation-impart of Serial No.
and then permitted to dry.
The dried product is extremely resistant to corrosion,
277,286, ?led March 18, 1952, now Patent No. 2,768,
103, granted October 23, 1956; Serial No. 278,481, ?led
particularly if coated with an acrylate lacquer or even 65 March 25, 1952, now Patent No. 2,768,104, granted Oc
a thin ?lm of methylmethacrylate resin. Other resins
tober 23, 1956; and Serial No. 371,427, ?led July 30,
1953, now Patent No. 2,777,785, granted January 15,
such as those made with ester-type Waxes, carnauba wax
for example, are also very effective.
Obviously many modi?cations and variations of the
The oxide-coated steel of the present invention is fur
ther protected against corrosion by applying to the coat 70 present invention are possible in the light of the above
ing a ?lm of an oil such as a para?in or a glyceride oil.
teachings. It is, therefore, to be understood that within
Thus mineral oil or palm oil can be applied in very min
the scope of the appended claims the invention may be
ute quantities (0.5 milligram or more per square foot)
practiced otherwise than as speci?cally described.
as by conventional electrostatic coating techniques and
What is claimed is:
enhances the corrosion resistance of the oxide coatings as 76
1. A plain carbon steel the surface of which is etched
With ammonium persulfate, the etched surface having a
corrosion-inhibiting coating of an in situ formed combi
nation of hydrated chromium oxides containing about 20
to 60% chromium by weight, about 40 to 95% of the
chromium by weight being trivalent, the remainder being
hexavalent, and the coating weighing about 10 to 200
milligrams per square foot of surface that it covers.
2. A plain carbon steel the surface of which is picric
acid etched; the etched surface being covered with a cor
rosion-inhibiting coating of an in situ formed combina
tion of hydrated chromium oxides containing about 20
to 60% ‘chromium by Weight, about 40 to 95% of the
chromium by Weight being trivalent, the remainder being
hexavalent, the coating weighing about 10 to 200 milli
grams per square foot of surface that it covers.
References Cited in the ?le of this patent
Hothersall ___________ __
Phelps ______________ __
ShOttOn _____________ __
Schuster et a1 _________ __
Kaercher et al. _______ __
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